The Collaborative International Dictionary of English v.0.48:

Sagebrush \Sage"brush`\, n.
   A low irregular shrub (Artemisia tridentata), of the order
   Compositae, covering vast tracts of the dry alkaline
   regions of the American plains; -- called also sagebush,
   and wild sage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sagebrush
    n 1: any of several North American composite subshrubs of the
         genera Artemis or Seriphidium [syn: sagebrush, sage
         brush]